I would think the casting/date code tells us what we need to know. a '55 date code would indicate a low water pump.... unless ... the casting code indicates it was cast in a '54 mold. If my '59 261 is any guide, the later crate engines made for the '54 and earlier trucks (i.e., "High water pump" engines) would have the '54 casting code but with the later date codes. Meaning that if both your casting and date codes indicate a '55 or newer 235 or 261, then you would have the low water pump.
I could be wrong about this, but I doubt it. I'm never wrong. A touch shy of being right sometimes, but never wrong.
